MSAN: Increase the number of shards to use for browser_tests.
MSAN: Increase the number of shards to use for browser_tests.

As is, the bots cannot finish running the tests in time.

BUG=581583

Review URL: https://codereview.chromium.org/1647713003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#371991}

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